1. Field of the Invention
The present invention relates to a process for producing crystalline ceric oxide particles. The ceric oxide is used as an abrasive, ultraviolet absorbing material, catalyst material, fuel cell material and the like. Out of these, the crystalline ceric oxide of the present invention provides an excellent material for use as an abrasive and an ultraviolet absorbing material.
Further, the present invention relates to the modification of the surface of a ceric oxide particle or a particle essentially composed of ceric oxide obtained by calcining and grinding or a composition containing a rare earth element essentially composed of cerium, as well as to an abrasive comprising the surface modified particles and a polishing method.
2. Description of the Related Art
Japanese Laid-open Patent Application No. Sho 63-502656 discloses a process for producing ceric oxide particles having a particle diameter of 0.05 to 10 xcexcm by holding an aqueous solution of cerium (III) nitrate or cerium (IV) nitrate in a sealed container at a temperature of 200 to 600xc2x0 C. and at a pressure of not less than 40 atm.
Japanese Laid-open Patent Application No. Hei 6-2582 discloses a process for producing crystalline ceric oxide particles having a particle diameter of not more than 300 angstroms which comprises cleaning a gel obtained by reacting a cerium salt compound and an alkali metal hydroxide or ammonia to remove impurities, adding an acid such as nitric acid or acetic acid to the gel, and subjecting the resulting mixture to a hydrothermal treatment at 100xc2x0 C. or more.
Japanese Laid-open Patent Application No. Hei 8-81218 discloses a process for producing ceric oxide particles having a particle diameter of 0.03 to 5 xcexcm which comprises adjusting pH values of a solution comprising ceric hydroxide and a nitrate to 8 to 11 using an alkaline substance and heating the solution at a temperature of 100 to 200xc2x0 C. under pressurization.
It is known that cerium is an element which is easily oxidized from valence (III) to valence (IV) in lanthanoids. For example, it is described on page 348 of xe2x80x9cInorganic Chemistry Vol. 1xe2x80x9d written by Thunderson and published by Hirokawa Shoten on Apr. 25, 1982 that cerium (IV) is produced when an alkaline suspension of cerium (III) hydroxide is exposed to air.
Since a hydrothermal treatment is carried out at a temperature of not lower than 100xc2x0 C. under such a condition that a corrosive substance such as nitric acid or acetic acid is contained in all the processes of Japanese Laid-open Patent Application Nos. Sho 63-502656, Hei 6-2582 and Hei 8-81218, a high-pressure container that meets the reaction conditions is required and further acid-resistant Teflon, glass or a corrosion-resistant alloy such as hastelloy must be used as a material for the high-pressure container.
By the way, it has been proved that cerium oxide particles or the particles of a composition essentially composed of cerium oxide have excellent performance as an abrasive for polishing inorganic glass, quartz crystal and quartz glass.
Japanese Laid-open Patent Application No. Sho 58-55334 discloses a process for producing a dispersible product containing a cerium compound which comprises heating a cerium (IV) oxide hydrate in the presence of a salt and disintegrating agglomerated fine crystals contained in the cerium (IV) oxide hydrate. The above publication teaches that ammonium nitrate is used as the salt in addition to a metal nitrate, metal chloride and metal perchlorate. The publication further teaches that a solution containing a cerium (IV) oxide hydrate and ammonium nitrate was dried at 105xc2x0 C. and further heated at 300xc2x0 C. to obtain a product containing ceric oxide and a nitrate which was then dispersed in water to obtain a gel, as an embodiment of the invention. However, the description is not made about the application of this gel.
Japanese Laid-open Patent Application No. Hei 5-262519 discloses a process for producing a rare earth oxide which comprises mixing an oxalate, rare earth compound and ammonium salt in an aqueous medium, separating a precipitate produced at 30 to 90xc2x0 C. and calcining the obtained oxalic acid rare earth ammonium double salt at 600 to 1,200xc2x0 C. The above publication teaches that ammonium nitrate, ammonium chloride, ammonium acetate or the like is used as the ammonium salt and cerium nitrate is used as the rare earth compound.
It is an object of the present invention to provide a process for producing crystalline ceric oxide from a cerium (III) salt based on a reaction mechanism that cerium (IV) is produced when an alkaline suspension of cerium (III) hydroxide is exposed to an oxidant such as air, by which crystalline ceric oxide particles having a particle diameter controlled to any value within the range of 0.005 to 5 xcexcm are produced by controlling the nucleus generation and crystal growth speeds of the crystalline ceric oxide particle. As the crystalline ceric oxide particles are produced at normal pressure (atmospheric pressure) in this process, a bulky high-pressure container is not necessary, thereby making it possible to produce crystalline ceric oxide particles by safe operation at a low cost.
A silicon oxide film (SiO2 film) for a semiconductor device, quartz glass for a photo mask and a quartz piece for a crystal oscillator are becoming in need of a polished surface having high flatness and accordingly, an abrasive having a particle diameter in a submicron or lower order must be used.
When the particle diameter of an abrasive used for polishing these is reduced, mechanical polishing force is lowered, thereby lowering the polishing speed and productivity in the polishing step with the result of an increase in product cost. If the polishing speed can be increased without changing the particle diameter of an abrasive particle, the productivity of the polishing step will improve.
When a silicon oxide film (SiO2 film) is used as a stopper (layer for stopping polishing) for flattening an interlayer film for a semiconductor device, if only a soft film such as an organic resin film can be polished without polishing a hard film such as a silicon oxide film (SiO2 film) so that polishing of a soft film such as an organic resin film proceeds while polishing of a hard film such as a silicon oxide film (SiO2 film) stops, the application of an abrasive to lithography is expected.
It is another object of the present invention to provide an abrasive comprising ceric oxide particles or particles essentially composed of ceric oxide prepared by heating ceric oxide particles or particles essentially composed of ceric oxide obtained by calcining and grinding a composition containing a rare earth element essentially composed of cerium in an aqueous medium at a temperature of 50 to 250xc2x0 C. in the presence of an ammonium salt. This abrasive is composed of surface modified ceric oxide particles. The polishing speed can be controlled and a polishing method suitable for the properties of a surface to be polished can be adopted for an abrasive composed of ceric oxide particles obtained according to type of a chemical used for surface modification.
According to a first aspect of the present invention, there is provided a process for producing crystalline ceric oxide particles having a particle diameter of 0.005 to 5 xcexcm (micrometer), which comprises the steps of:
reacting a cerium (III) salt and an alkaline substance in an (OH)/(Ce3+) molar ratio of 3 to 30 in an aqueous medium in an inert gas atmosphere to produce a suspension of cerium (III) hydroxide, and
immediately blowing oxygen or a gas containing oxygen into the suspension at a temperature of 10 to 95xc2x0 C. and at an atmospheric pressure.
According to a second aspect of the present invention, there is provided an abrasive composed of ceric oxide particles which are heated at a temperature of 50 to 250xc2x0 C. in an aqueous medium in the presence of an ammonium salt.
According to a third aspect of the present invention, there is provided a polishing method using an abrasive composed of ceric oxide particles which are heated at a temperature of 50 to 250xc2x0 C. in an aqueous medium in the presence of an ammonium salt.